1. Field of the Invention
The disclosure relates to an electrically conductive material, and particularly relates to an electrically conductive material with nanostructures.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
The 3C products with characteristics of light weight, thin profile, small size, softness, and flexibility are required in this century. According to the IDTechEX 2005 statistical report, the market for flexible electronics will reach to US$4.75 billion in 2010. The report also predicts that the flexible electronics market will grow to US$250 billion before 2025. The report reveals that a conductive composition, which can be cured at a low temperature and formed into a conductive film with high adhesion force for attaching to a flexible substrate, is necessity. NanoMarkets in 2006 estimated that the market for conductive inks will grow from US$1.10 million to US$1.36 billion between 2007 and 2010. It can be seen that the development of conductive inks can effectively enhance the development of new flexible electronics and flexible sensor industries. In addition, conductive inks can be used for forming patterned conductive layers on substrates using an inkjet or screen printing technique which does not need complicated lithographic processes, resist-development processes, or vapor deposition processes. Therefore, our processes can meet the world trend of saving energy, reducing carbon generation, and lowering cost.
Conductive inks include conductive fillers, which can be metal or alloy material such as gold, silver, copper, iron and aluminum. When the size of the conductive filler is reduced in nanoscale, the surface area of the conductive filler may increase sharply. This could cause the melting point of the conductive fillers to decrease and make the conductive fillers easier to anneal to each other. For example, silver has a melting point of 690 degrees Celsius. When the size of silver nanoparticles is reduced to 2 nanometers, the melting point of the silver nanoparticles may decreases to around 100 degrees Celsius. Currently, the most popular conductive filler on the market is silver nanoparticle filler. Compared to other metal nanoparticles, silver nanoparticles have advantages of low price, high electrical conductivity, lower annealing temperature, and high stability. In addition, silver oxide nanoparticles are conductive. Therefore, silver nanoparticles are widely applied. Copper nanoparticle filler is another low cost material with high electrical conductivity compared to that of silver nanoparticles; but copper nanoparticles are easily oxidized, and therefore have a limit in application.
Conductive ink comprises conductive particles, a linker, and an auxiliary agent. When a conventional conductive ink is coated on a substrate using inkjet or screen printing technique, the electrical conductivity of the conductive film must be annealed at a high temperature to remove the linker, solvent, and other mixing materials so as to increase its conductivity. This high temperature heating process could damage the substrate and other materials because of their different thermal expansion coefficients. In particular, when a substrate is made of materials with different coefficients of thermal expansion, it could cause the thermal cracks. Furthermore, this heating process also could damage a soft substrate having a low melting point, for example a polyethylene terephthalate substrate or a polymethyl methacrylate substrate.
U.S. Patent Publication No. 2007/0,074,316 discloses a transparent conductor including a substrate and a plurality of silver nanowires. The silver nanowires form a network structure embedded in a matrix so as to form an optically transparent and electrically conductive layer.
U.S. Pat. No. 7,341,680 discloses a printable composition with two types of materials. The first type of material can be of wire shape, conical shape, round shape, or plate shape, and can have an aspect ratio greater than 1:3, and the second type of material can be of round shape, elliptical shape, cylindrical shape, or pyramidal shape, and can fill into the pores of the first type of material. This electrically conductive film composition exhibits better flatness and higher conductivity.
In summary, conventional conductive layers fabricated using electrically conductive inks or pastes having nanoparticles require a high temperature sintering process likely to cause unwanted consequences, and the electrically conductive layers fabricated using inks having nanowires may have issues of low density and poor flatness and the electrical conductivity thereof cannot be further increased. Therefore, there is a need to develop a conductive layer having good flatness and high density without requiring a high temperature annealing process, and a method for manufacturing the same.